I had to replace mine recently. The original size wasn't available so I got one that was slightly larger. It fits in the battery box, but the cover presses tightly against the top.
The almost exact replacement is: SLA 1005 it is 1.3 Amp /hour. The original specs are available from Whizzer also.
By the way, the Whizzer doesn't like to keep the battery charged.
Hope this helps.
PS: The dimensions for the slightly larger battery are: 3 -5/8" (9.6 cm) long or wide x 1- 15/16" height (5 cm) x 1- 5/8" depth (4.5cm). The battery sits "sideways" in the battery box, its terminals face outboard.
The dimension that doesn't quite fit is the height at 1-15/16", I would suggest 1-13/16" or smaller.
PPS: If you need, I can ship two from here.